The present invention relates to a process for impregnating porous carbon bodies for protection against oxidation and, particularly, to a process for impregnating porous carbon bodies, namely, carbon fiber-reinforced carbon bodies, for protection against oxidation. The carbon bodies are treated at room temperature under vacuum with a low viscosity gel and/or sol, formed from refractory and/or non-combustible inorganic compounds, as antioxidant, in an excess, with respect to the open pore volume which is present in the carbon body and is to be lined. The open pores in the carbon bodies are penetrated by the antioxidant and the surfaces of these pores and the outer surfaces of the carbon bodies are wetted with a coherent coating of the antioxidant, after which the gel and/or sol is allowed to dry at room temperature, the vacuum being removed to atmospheric pressure.
Both because of their good technical characteristics, such as resistance to friction at high temperatures, and also because of their acceptability from environmental and health standpoints, bodies consisting of carbon, in particular carbon fiber-reinforced carbon bodies, have already found extensive use in industry.
A disadvantage of these materials is that they have low resistance to oxidation by atmospheric oxygen at relatively high temperatures, i.e. at temperatures of as low as 350.degree. to 500.degree. C. If these materials are used, for example, as friction material, they are continuously exposed to wear at their surfaces. Protection against oxidation by oxygen can therefore not be achieved merely by applying a protective layer on the surface of the carbon bodies, as is customary in the case of carbon components in the aerospace industry. In the case of a carbon body used as friction material, the entire body, that is to say including the interior of the material, must accordingly be treated homogeneously with the antioxidant.
There has been no lack of attempts to eliminate this disadvantage of oxygen sensitivity in the case of carbon bodies, which otherwise, because of their good technical use characteristics, can be used in diverse applications, including friction material.
Thus, German Offenlegungsschrift No. 3,005,902 describes protection of carbon-containing components of metallurgical aggregates against oxidation by forming a glassy mass in the pores, first by saturating the component with orthophosphoric acid, then drying it, subsequently resaturating the component with an organometallic compound and heating the re-saturated component to the temperature for phosphate formation.
German Offenlegungsschrift No. 3,731,540 describes a friction material in which the pore regions of a composite material, which comprises a carbon fiber-reinforced carbon material, have been infiltrated by a metal.
German Offenlegungsschrift No. 3,622,437 describes a friction element consisting of a carbon-carbon composite material, which, in order to improve the oxidation characteristics, also contains one or more nitrides of the elements of group IVb of the Periodic System of the Elements.
German Offenlegungsschrift No. 2,600,169 describes a process for the production of carbon coatings on carbon materials or metals in which refractory or non-combustible substances are introduced into the fine carbon layer to be applied. These substances are specifically, acrylamide polymers which are pyrolyzed.
German Offenlegungschrift No. 3,426,911 describes a carbon-carbon composite article which consists of a carbon-carbon substrate, a SiC coating and a layer of Si.sub.3 N.sub.4, applied by chemical vapor deposition (CVD), on the outer surface of the SiC layer in order, inter alia, to eliminate the sensitivity of the carbon materials towards oxidation.
German patent No. 3,942,025 discloses a protective layer for the prevention of oxygen embrittlement of titanium components, which layer consists of Na water-glass varnish and silicon powder.
German patent No. 4,003,627 describes the use of a sol/gel process for glazing moldings consisting of at least latent hydraulic binder systems. In this process, the surface of a hardened molding is coated with a SiO.sub.2 containing glaze slip in dissolved or colloidal form. It is also possible for the slip to contain boron or sodium tetraborate.
U.S. Pat. No. 3,706,596 discloses a process to increase the oxidation stability by treating carbon materials successively with a colloidal dispersion of finely divided silica and an impregnating solution of methyl borate and methyl phosphate, so that a SiO.sub.2- B.sub.2 O.sub.3- P.sub.2 O.sub.5 glaze forms on the accessible surfaces.
U.S. Pat. No. 4,863,773 discloses a composite material containing carbon fibers, in which material the individual carbon fibers are coated with a layer of SiC and these are embedded in a material consisting of C/SiC. SiC layers, which are surrounded by silica and a SiO.sub.2- B.sub.2 O.sub.3 glass layer, are present as a surface seal.
These known processes for protecting carbon bodies against oxidation by atmospheric oxygen are in some cases very expensive and do not always meet the expectations.
An object on which the present invention is based is to obtain, by way of a technically simple process, good protection against oxidation by the ambient atmosphere in the case of carbon bodies having very diverse specification profiles. The aim is that the carbon body should have good friction characteristics when used as friction material and an optimum ratio between coefficient of friction and wear should be maintained.
The foregoing object has been achieved according to the present invention by a process in which the carbon body is treated with SiO.sub.2- B.sub.2 O.sub.3 gel and/or sol having a molar composition of SiO.sub.2 to B.sub.2 0.sub.3 of 60:40 to 85:15 as the antioxidant, the treatment of the carbon body with the anti-oxidant is carried out at a temperature of 15.degree. to 25.degree. C. under a vacuum of 10.sup.-2 to 10.sup.-4 bar, and subsequently, the carbon body treated in this way, with the antioxidant which has infiltrated the pores and has been applied to the surfaces, is subjected to a multistage temperature treatment. The carbon body, after the drying operation in air, is first further dried for up to 5 hours at 110.degree. to 130.degree. C., then is kept at a temperature of 200.degree. C. for up to 2 hours and then kept at a temperature of up to 600.degree. C. for a period of 2 to 10 hours.